Modern primary cells, such as lithium/iodine and lithium/chlorine cells are very efficient power sources for applications where a low output power is required. Amongst uses of such cells there may be mentioned their use as power source of medical devices such as tissue stimulating devices, and more particularly cardiac pacemakers of the implantable type. This type of cells is characterized by a very low internal leakage, chemical stability and excellent energy to volume ratio.
Amongst the drawbacks of this type of cells there may be mentioned the decrease of voltage and increase of internal impedance upon cell depletion and the relatively high specific gravity. Thus, it is desirable to use a single cell in order to decrease the overall weight of the device.
The output voltage of a single cell is too low for the intended purposes. For stimulating cardiac muscles a voltage of at least about 3.5 V is required in order to provide reliable stimulation. On the other hand the energy required is quite low, of the order of about 50 microjoules per pulse.
In order to be able to use such cells with this type of device, it is necessary to provide means for stepping up the voltage, supplied to the device. The output must be of predetermined elevated value, combined with high efficiency and stability versus cell voltage and load variations.
There exist various DC/DC conversion circuits for such applications. One of the more widely used ones is based on oscillator means combined with a low resistance starting current path. This type of device makes use of a high frequency oscillator which provides a comparatively high open circuit output voltage which is reduced by a serial type stabilizer resulting in substantial energy losses. Upon actuation a high current is drawn from the cell and there exists the danger of stoppage of oscillations in case of overload, resulting in possible cell depletion. In case of oscillator failure current supply to the device actuated is stopped and this is unacceptable with critical devices such as cardiac pacemakers.
Another conventional device comprising oscillator means, such as transformer coupled multivibrator has similar problems of voltage stabilization.
Switching type voltage multipliers are less efficient and as the output voltage is a multiple of the cell voltage, the output voltage varies according to the decrease of cell voltage upon cell depletion. Another disadvantage of the conventional DC/DC regulated systems is the use of zener diode as a voltage reference. Zener diode requires high current drain, of at least 50 .mu.A, to allow reasonable voltage reference while for modern cardiac pacemaker the overall current drain is less than 20 .mu.A including the output energy for heart stimulation.